Almost all of the tissue of a mammalian body comprises a fine mesh of very thin blood vessels, each of which is thinner than a human hair. Usually, neither the number nor the size of these vessels increase, since the division of the endothelial cells covering the vessels is slow, actually up to several years. The exceptions are for example during wound healing and menstruation, when the vessels grow rapidly. However, that is during a limited period of time and the cell division ceases thereafter.
The generation of new blood vessels from existing ones is called angiogenesis. Angiogenesis has been associated to cancer and the formation of tumors as well as to other conditions, such as diabetes retinopathy, rheumatoid arthritis and even some inflammatory conditions. Accordingly, a considerable research effort is made world-wide to find ways of preventing and inhibiting the angiogenic process. If this were possible, tumor growth could be controlled and useful therapies could be developed regarding the above mentioned conditions.
There exists numerous pieces of evidence showing that tumors are depending on de novo formation of blood vessels for expansion beyond a mass of a few mm3. The angiogenesis is triggered by factors secreted by the tumor cells. It has recently been discovered that tumors through unleashed proteolytic activity generates peptide fragments, which show anti-angiogenic activities. One example is the molecule angiostatin, which is a fragment of plasminogen.
Plasminogen is a substance in blood plasma which, when activated, forms plasmin or fibrinolysin, an enzyme involved in the coagulation of blood. Plasminogen itself lacks any detectable anti-angiogenic activity. It has been found (Judah Folkman et al, Harvard Medical School, Boston) that a part of this endogenous protein, more specifically the first four kringle domains, is capable of preventing the endothelial cells from dividing. This part of plasminogen has been denoted angiostatin, and a great deal of research within this field is centred around this molecule. The prior art has shows that angiostatin inhibits endothelial cells specifically in vitro and blocks angiogenesis in vivo. Systemic treatments with subcutaneous injections of angiostatin induces in vivo dormancy in a wide range of tumors in SCID mice (O'Reilly et al., Nature Med., vol. 2, p. 689, 1996). No detectable toxicity has been detected in these animals even after months of treatment. Angiostatin shows two levels of specificity: it induces apoptosis specifically in endothelial cells in vitro (Claesson-Welsh et al., Proc. Natl. Acad. Sci., USA, vol. 95, p. 5579, 1998) and only affects endothelial cells active in angiogenesis in vivo. It has not shown to negatively affect cells in established vessels.
Angiostatin does indeed exhibit some advantageous properties, inter alia as it is an endogenous substance. However, the disadvantages associated with its possible use for medical purposes cannot be neglected. One is that the half life thereof is very short, it may be counted in hours, thereby requiring a frequent administration thereof. This far, the efficiency thereof has proven to be rather low, which fact necessitates the use of large doses thereof. These two disadvantages are in themselves strong motives for directing further research towards the finding of alternative, smaller and/or more efficient molecules to be used as medicaments.